implanted into bonefish. Each tag emitted a series of ultrasonic
pings unique to that fish, which were detected and recorded by
an array of underwater acoustic receivers. By placing acoustic
receivers around the possible spawning site at the southern end
of Abaco, as well as along the potential shoreline travel corridor,
it was possible to identify long-distance spawning movements
without ever having to physically recapture tagged fish.
Acoustic tagging of Abaco’s bonefish began in October of 2010.
While the team of researchers had hoped to find the schools of
fast-moving bonefish that were rumored to run southward along
the outside edges of The Marls, a week of searching revealed no
migrating fish. Instead, 14 fish were captured and
tagged deep within The Marls. Then it became a sit-
and-wait game while the tagged bonefish did their
thing. When the underwater receivers were finally
pulled to the surface and downloaded the following
May, we had the data we were hoping for—one of
the fish that had been tagged in The Marls the
previous October had migrated all the way to the
potential spawning area at the south end of Abaco,
a round trip distance of at least 100 miles! What’s
more, the fish hung around that area from December
1-3, just two days before the new moon, which
corresponds with earlier observations made in
Eleuthera and Andros. But one fish just isn’t enough
to tell the whole story...
After observing a tagged bonefish make a potential
100+ mile spawning run in 2010, the BTT
researchers returned to Abaco in the fall of 2012 to
determine if large numbers of bonefish were also
making this migration, and to confirm that the
migration was all about spawning. Instead of tagging fish deep
within The Marls as they had done in 2010, the team hit the
jackpot in 2012, intercepting school after school of bonefish
running south along the outer edge of The Marls.
The scientists surgically implanted acoustic tracking tags into 25
of these migrating fish. They also dart-tagged 600 additional fish
from the fast-moving schools, hoping that guide-based recaptures
in the Marls would link the migration corridor with home range
habitats. In the final few hours of the final day of the 2012
research trip, the BTT team made an incredible discovery. While
deploying underwater acoustic receivers near the likely spawning
area, the scientists stumbled onto a school of bonefish that was
at least 10,000 to 20,000 fish strong! The size of the school,
along with the way the fish in it were behaving, convinced the
researchers that they had found a pre-spawning aggregation.
The following spring, after the receivers were recovered, the BTT
team was all smiles as they downloaded the data from the
underwater receivers. Six of the acoustically tagged fish had
migrated from The Marls to the potential spawning site in
December 2012. The fish arrived between December 10th and
December 16th, and stayed for 2 to 5 days. The new moon that
year was on December 13th, right in the middle of the migration.
One of the tagged fish made a second run to the south end of the
island between March 6th and March 11th. It was no surprise
to see that this run matched up with a March 11th new moon.
As all of the pieces came together, it became pretty clear thatbonefish were making multiple 100+ mile round-trip spawning
runs from The Marls to a very specific spot at the southern end
of the island each fall and winter.
With the discovery of a likely pre-spawning aggregation and the
identification of multiple migrations between The Marls and the
southern end of Abaco, BTT researchers returned to the
aggregating site in the fall of 2013 to develop a better
understanding of pre-spawning and spawning behavior in
bonefish. The team used real-time acoustic telemetry (“active
tracking”) to follow fish within pre-spawn and spawning schools.
With a directional receiver mounted to the bow of their boat, the
team tracked the location and depth of acoustically tagged fish
in the aggregating schools day and night for a week. They
discovered that pre-spawning schools, numbering in the
thousands of fish, would stage in 3 to 6 feet of water for several
days prior to the spawn. As the full moon approached, these
staging schools grew larger.
On the afternoon before the full moon, a huge school of bonefish
—estimated to contain more than 10,000 fish—moved offshore
from the shallow staging area into adjacent waters in the 10 to
30 foot depth range. These fish began to show the same pre-
spawning behaviors that had been observed in Eleuthera—a hint
that they were getting ready to make the run offshore to spawn.
As the huge school swirled into deeper water like a living tornado,
individual bonefish began bumping and nudging into each other.
Just before sunset, the fish began porpoising at the surface,(left) Underwater acoustic receivers
were deployed near the potential
spawning site to “listen for” tagged
bonefish. Photo by Dr. Zack Jud(above) Surgically implanting
an acoustic tracking tag into
a migrating bonefish.
Photo by Buddy Pinder(left) Real-time acoustic
tracking allowed researchers
to follow tagged bonefish as
they moved offshore to
spawn. Photo by
Alex Lovett-Woodsum